The present invention relates generally to switch-mode power supplies, and specifically to switch-mode power supplies operating from low voltages.
Power converters or power supplies are well known subjects to electronics engineers. In automotive applications where the vehicle chassis forms a ground reference, power supplies for electronics must operate from battery voltage which can get as low as 7 volts when the ambient air temperature is below freezing, even down to 3.5V when a starter is activated. Most modem electronic circuits operate on standard voltages such as +12V, +5V, and xe2x88x9212V. Therefore a buck-type, as depicted in FIG. 1A, or boost-type, as depicted in FIG. 1B, switch-mode power supply (SMPS) cannot be used alone in automotive applications. A flyback SMPS, as depicted in FIG. 1C, can be used, only to supply relative low power levels. As a result of low minimum battery voltages, the current level that a SMPS has to deal with can be very high, in the tens of amperes for an output power of just about 50-100 watts. Conventional approaches to SMPS, such as the forward converter taught in U.S. Pat. No. 4,870,555, or the push-pull converter taught in U.S. Pat. No. 5,140,509, or a flyback SMPS result in high losses in the MOSFET switches, and in the snubber networks, even in the so-called xe2x80x9closslessxe2x80x9d snubbers or active core reset circuits. In such a lossless snubber, a current of 20 amperes through a Schottky rectifier would result in over 5 watts of heat dissipation in that rectifier. MOSFETs switching such a high current will dissipate similar heat quantities. Therefore the requirements of high efficiency, small size and low cost may not be easily met. These requirements apply even more relevantly to portable equipment operating from lower voltage battery such as portable camera and boom boxes, and home/office equipment such uninterruptable power supplies (UPS) and battery-operated appliances.
Thus there is a need for a better approach to power supplies for automotive, portable and mobile applications, or in general for battery-operated electronics, especially when the power demand is over 50 watts. Such power demand is common for some high power audio amplifiers, inverters, and computer-controlled systems.
The invention provides a family of high power converters operating from low voltages. This family of converters essentially replaces switches and transformer windings carrying high currents in opposing directions with switches and windings processing the difference of those high currents, an approach that results in very substantial reductions in conduction and switching losses, as well as reduced losses in auxiliary circuits such as snubber networks and core reset circuits. Some of the transformers used in the various embodiments only have a tapped winding conducting only the difference of currents, therefore they are very small compared to a conventional multiple-winding transformer processing the same power, each winding conducting much higher current. In some of the embodiments, the modulator sequences the drive signals to the switches in such a way that switches carrying heaviest currents switch in zero current switching.
Accordingly, several objects and advantages of this invention are:
(a) to provide a method and apparatus to supply regulated power to battery-operated electronics
(b) to provide a method to minimize the size of high power SMPS operating from low voltage
(c) to provide a method for minimizing component count in SMPS
(d) to provide a method and apparatus for reducing switching losses by zero current switching.
Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing description.